In an exposure apparatus, an inspection apparatus, a machine tool, and the like, a positioning apparatus is used to position an object to be processed, or the like. FIG. 8 is a perspective view showing the schematic structure of a positioning apparatus formed as a wafer stage device for a semiconductor exposure apparatus. A wafer stage device 100 has a wafer chuck for holding a wafer (substrate). During exposure, the wafer is stepped to sequentially transfer a pattern onto the respective exposure regions on the wafer. The wafer stage device must have an accurate, high-speed positioning performance for forming a fine pattern and improving throughput.
To satisfy this demand, as shown in FIG. 9, the wafer stage device 100 is formed by combining a coarse movement portion 101 and a fine movement portion 102. The fine movement portion 102 is mounted on an X-Y slider 103. A hydrostatic guide (not shown) is arranged on the lower surface of the X-Y slider 103 to allow the X-Y slider 103 to move smoothly on a surface plate 104. An X beam 105 and a Y beam 150 extend through the X-Y slider 103 to transmit thrusts in X and Y directions to the X-Y slider 103. The X beam 105 and Y beam 150 are perpendicular to each other. The X beam 105 is hydrostatically guided by an X yaw guide 151 fixed to the surface plate 104. This regulates inclination of the X beam 105 in a yaw direction within an X-Y plane, so that the X beam 105 can move only in the X direction. The Y beam 150 is hydrostatically guided by a Y yaw guide 152 fixed to the surface plate 104 to be perpendicular to the X yaw guide 151. This regulates inclination of the Y beam 150 in the yaw direction within the X-Y plane, so that the Y beam 150 can move only in the Y direction.
The thrusts are supplied to the X beam 105 and Y beam 150 by X-movement linear motors 110a and 100b, arranged at the two ends of the X beam 105, and Y-movement linear motors 111a and 111b, arranged at the two ends of the Y beam 150, respectively. Each of the linear motors 110a, 110b, 111a, and 111b includes a movable element and a stator. Magnets, serving as the movable elements, are fixed to the two ends of each of the beams 105 and 150. When the movable elements are moved, the corresponding beams can be moved.
The positional relationship among the X beam 105, Y beam 150, and X-Y slider 103 will be explained by way of the positional relationship between the X beam 105 and X-Y slider 103.
FIGS. 10 and 11 are a (partially cutaway) plan view and side view showing the structure of the X beam 105 and X-Y slider 103. FIG. 12 is a perspective view of a coil unit 108 (to be described later). As shown in FIG. 12, the coil unit 108, obtained by winding a coil 107 on a core 106 (to be referred to as an E core hereinafter), is fixed to the X-Y slider 103. The E core 106 is obtained by stacking silicon steel plates, each having an E shape. In the example shown in FIG. 10, a total of four coil units 108a, 108b, 108c, and 108d are fixed to the X-Y slider such that two units on either side of the X beam 105 sandwich it.
Rectangular parallelepiped cores 109a and 109b (to be referred to as I cores hereinafter), each obtained by stacking silicon steel plates, are fixed to the two side surfaces of the X beam 105 within the movable range of the X-Y slider 103.
As shown in FIG. 12, a gap h is formed between the E core 106 and the corresponding I core 109. When a current is supplied to the coil 107, a magnetic circuit is formed between the E core 106 and I core 109 to generate an attracting force. The gap h can be changed by using the attracting force to control the positions of the X beam 105 and X-Y slider 103 relative to each other.
The pair of coil units 108a and 108b and the pair of coil units 108c and 108d are arranged on the two sides of the X beam 105, such that the lines of operation of the attracting forces substantially coincide with each other. Thus, the X-Y slider 103 can be moved in two directions, i.e., to the + and − sides in the X direction.
Gaps h1, h2, h3, and h4 (see FIG. 10) are detected by sensors (not shown). While controlling currents to be supplied to the coil units 108a, 108b, 108c, and 108d, on the basis of detection information obtained by the sensors, the X beam 105 is moved by the X-movement linear motors 110a and 110b. Thus, the X-Y slider 103 can be moved, while it is kept to not be in contact with the X beam 105.
This structure applies to the gap between the Y beam 150 and X-Y slider 103. When the X beam 105 and Y beam 150 are moved independently of each other, the X-Y slider 103 can be moved in the X and Y directions on the surface of the surface plate 104.
Usually, during operation, the X-Y slider 103 is moved while maintaining the X beam 105 and Y beam 150 to not be in contact with the X-Y slider 103. To move the X-Y slider 103 by driving the beams 105 and 150, while maintaining the gaps h between the I cores 109 on the X beam 105 side and Y beam 150 side and the E cores 106 on the X-Y slider 103 side, the currents to be supplied to the E cores 106 must be controlled highly accurately.
During operation before a stage, when a highly accurate control operation is performed, as in assembly, during a test operation in maintenance, or when unexpected disturbance occurs, sometimes, the non-contact state between an I core 109 on a beam side and an E core 106 on the X-Y slider 103 side cannot be maintained, and the I core 109 and E core 106 may undesirably come into contact with each other. When this contact occurs, it may form a hitting mark on a pertinent component or cause wear. Then, the gap h between the I core 109 and E core 106 may change locally, or the upper surface processed by plating, or the like, may be damaged, to cause the I core 109 or E core 106 to rust, eventually, impairing smooth movement of the X-Y slider 103. This interferes with accurate positioning. As fluctuations in gap h between the I core 109 and E core 106 can cause the X-Y slider 103 to rotate in a yaw direction (ωZ direction), the edge of the E core 106 may come into contact with the I core 109, to promote the damage. Furthermore, if the X-Y slider 103 moves with I core 109 and E core 106 being in contact with each other, the damage can become more apparent.